In recent years, chip-type electronic parts used for surface mounting, such as IC's, transistors, diodes, condensers, piezoelectric resistors and the like have been supplied by being contained in packaging materials consisting of a plastic carrier tape having at given intervals pockets formed by embossing so as to accommodate chip-type electronic parts of particular shape and a cover tape heat sealable to the carrier tape. The electronic parts contained in the packaging materials are automatically taken out after peeling the cover tape of the package and are mounted on the surface of an electronic circuit substrate.
In use, the carrier tape and cover tape are stored on separate roles or reels. The carrier tape is unwound from its storage reel and extended in a linear fashion, such that parts can be inserted into its cavities. As the parts are inserted, the cover tape is applied along the linear length of the carrier tape, such that an adhesive coated side of the cover tape comes into contact with the carrier tape. The cover tape and carrier tape are in contact with one another at their linear edges, and the adhesive on the cover tape is activated (rendered tacky) by the application of heat or pressure at those points in which it is in contact with the carrier tape. The heat is provided in a sufficient amount, balanced with an appropriate amount of pressure and dwell time, to activate the adhesive, such that a bond of uniform strength is obtained across the length of the tape. The heat and pressure can be applied by any one of a number of different techniques, e.g. hot air guns, drag shoes, ultrasonics, reciprocating sealing shoes, heated pinch rollers, etc. In several prior art devices, the adhesive carried on the cover tape that is not subjected to the heat and pressure is not activated, and as such, it remains non-tacky.
With the significant improvement in surface mounting techniques in recent years, electronic parts to be surfaced-mounted have come to be made in chips of higher capability and smaller size. In such a movement, the electronic parts, when transferred in a package have undergone vibration and contact with the embossed inner surface of the carrier tape of the package, or the inner surface of the carrier tape of the package, and the resulting friction generates static electricity. Static electricity has also been generated when the cover tape is peeled off from the carrier tape; this static electricity has generated sparks to cause destruction and deterioration of electronic parts. Thus, the electronic parts have had troubles due to static electricity, and it has been an important task to develop an anti-static measure for packaging materials, such as carrier tape and cover tape.
The anti-static treatment for a carrier tape has before been effected by incorporation or coating of carbon black into or on the carrier tape used, and the effect has been satisfactory. For the anti-static treatment of the cover tape, the coating of an anti-static agent or conductive material on the outer layer of the cover tape has been utilized. This treatment, however, has not been sufficient for the protection of electronic parts to be contained by sealing because the treatment is applied to the outer side of the cover tape, and particularly the treatment was ineffective for the static-electricity generated by the contact of the inner surface of the cover tape with the electronic parts. The anti-static treatment for the inner surface of the cover tape, i.e. the adhesion layer of the cover tape can be effected by coating or incorporation of an anti-static agent on or into the adhesion layer. In this approach, however, the anti-static agent incorporated into the adhesion layer has bled onto the inner surface of the cover tape and invited unstable sealing and many troubles due to poor sealing. Further, the anti-static effect has decreased with the lapse of time and has been greatly affected by the conditions under which the package was used, such as temperature and humidity, and has significantly decreased under a low humidity, such as 10 percent R.H. (relative humidity). Therefore, no sufficient effect has been obtained.
The incorporation of a conductive material into the adhesion layer has been technically difficult as well. This is because the adhesion layer has been formed by laminating a film or the like to an outer layer, and the incorporation has significantly reduced the transparency of the resulting cover tape, making the cover tape usability questionable. The coating of a conductive material on the adhesion layer has not been effected, because the selection of a binder that may stably bond to the carrier tape has been difficult and because the adhesion layer is covered and hidden by the coating.
An additional problem with respect to items enclosed by cover tapes in packaging systems is machine readability of the item. As industry has become more automated, light sensors have increasingly been used to identify orientation, quality, and quantity of products which are sealed in a carrier tape packaging system. The light sensors require a transparent cover tape in order to function properly. This transparency and hence, machine readability, can be significantly affected by adhesive materials on the cover tape.
Several prior art methods have been used to attempt to solve these problems. For example, U.S. Pat. No. 5,064,064 and sister U.S. Pat. Nos. 4,994,300 and 4,929,486, each issued to Itou et al. and disclose a device for packing chips with a cover tape for the same. The cover tape disclosed includes a base tape, an adhesive layer formed on only one surface of a base tape, and a non-adhesive layer with an anti-static agent, which is pattern-printed on the adhesive layer centrally along the longitudinal direction of the base tape. The anti-static properties of this tape are only located over the chip materials. Furthermore, the tape disclosed does not provide a clear package in which to view the chip or part located in the carrier tape beneath the cover tape. Similarly, U.S. Pat. No. 5,208,103 issued to Miyamoto et al. and U.S. Pat. No. 5,346,765 issued to Maeda et al. are each directed towards cover tapes for packaging chip-type electronic parts. Each of these patents utilizes a biaxially oriented film with an adhesion layer of a dispersion of conductive fine powders. These patents still require an adhesive layer across the entire carrier tape surface, therefore allowing the chip contained in the pocket below the cover tape to contact the adhesive.